Virtually all open water systems have problems with microorganisms such as bacteria and algae, and various treatments are used to eliminate the microorganisms. For example, in swimming pools it is common to add a source of chlorine to destroy microorganisms. Other water systems employ additional biocides and algicides. It is very common to add chlorine or ozone or other chemicals to drinking water to destroy microorganisms.
Environmental and health regulations have generally insured that chemical treatments are safe. However, due to increased sensitivity to environmental concerns, philosophically it is preferable to minimize or eliminate the use of chemicals for treatment of water, particularly where the water is either consumed or discharged into the environment.
One particular method of destroying microorganisms is ultraviolet irradiation. At approximately 254 nanometers, ultraviolet radiation will destroy most microorganisms. Ultraviolet irradiation is used to treat potable water. It has also been used to treat swimming pools.
Electrolytic treatment of brine solution is used to generate hypohalite species which is added to waste water to destroy microorganisms.
Cooling towers and similar water systems require a sophisticated combination of chemical treatments to overcome a variety of problems. Antiscalants, bactericides, and other chemicals must be added to the cooling tower water. High intensity ultraviolet irradiation can cause a breakdown of certain chemicals. Also, with a system such as a cooling tower, algae tends to grow on the portion of the system which is exposed to the environment. There is no good way to expose this surface to ultraviolet radiation to prevent algae growth. Other microorganisms also grow on areas such as heat exchangers which are totally inaccessible. Therefore, some biocide or algicide is required.
Also, in cooling towers, the heat generated by the ultraviolet light source is prone to cause scale formation on the U.V. lamp particularly with medium pressure lamps. This is in part due to the temperature, alkalinity and total hardness of the water. Turbity also interferes with U.V. light transmission. For these reasons, ultraviolet radiation has not been used to destroy microorganisms in open water systems.